The present invention relates to tantalum powder useful as a material for electrolytic capacitors, an anode body for electrolytic capacitors which is prepared by sintering the tantalum powder and an electrolytic capacitor in which the anode body is incorporated.
Tantalum has been used as an electrode material for capacitor because of a high dielectric constant, on the order of about 27, of the oxidized film thereof, but there has recently been desired for the development of an electrolytic capacitor having an improved capacity for the reasons of competition with other capacitors and a general tendency toward miniaturization of electric and electronic apparatuses and correspondingly, there has rapidly increased a need for tantalum powder as such an electrode material.
Tantalum powder is in general prepared by reducing potassium tantalum fluoride with sodium. The spault-like tantalum powder thus prepared includes salts as by-products and accordingly, the tantalum powder is subjected to a treatment such as water-washing and/or acid-washing to remove the salts present therein and to thus improve the purity thereof and then dried to give raw powder. The tantalum powder prepared through reduction with sodium metal is not easily handled since it comprises fine particles having an average particle size on the order of 0.3 to 3.0 .mu.m. For this reason, tantalum powder containing fine particles is heat-treated in a vacuum to once agglomerate the powder, followed by disintegrating the agglomerate into porous particles having a particle size ranging from 1.0 to 5.0 .mu.m and a large specific surface area prior to the practical use thereof.
As has been discussed above, tantalum powder (raw powder) is heat-treated in a vacuum to remove any gaseous components (hydrogen gas) and metallic impurities (such as sodium and potassium) and to granulate the fine particles and improve the flowability of the powder. Incidentally, the content of oxygen present in the tantalum raw powder having an average particle size ranging from 0.3 to 0.4 .mu.m ranges from about 7000 to 9000 ppm, but the content thereof is increased up to a level ranging from 12000 to 14000 ppm due to the heat-treatment in a vacuum. The presence of oxygen increases the leakage current of the powder which is an important electrical property. For this reason, most of the tantalum powder products are subjected to removal of oxygen through reduction with, for instance, magnesium metal to improve the leakage current, followed by acid-washing and drying processes prior to the practical use thereof in the electrode-production.
The electrostatic capacity (C) can be expressed by the following relation: EQU Electrostatic Capacity C=Dielectric Constant (.epsilon.).times.Specific Surface Area (S).div.Film Thickness (d)
Therefore, tantalum powder to be developed and used as an electrode material for electrolytic capacitors having an extra high capacity must have a specific surface area as large as possible. For instance, it is necessary, for the development of such tantalum powder, to prepare raw powder having a large specific surface area through reduction with sodium; to prepare raw powder whose impurity content is reduced to a lowest possible level so as to cope with the requirement for the pellet-sintering temperature which has recently been inclined to be reduced; and to select the optimum temperature for the heat-treatment so that the heat-treatment is not accompanied by a substantial reduction in the specific surface area of the raw powder.
However, when preparing tantalum powder having a large specific surface area and used as an electrode material for electrolytic capacitors having an extra high capacity according to a conventional method, the following problems arise.
1 Fine tantalum particles having a large specific surface area and prepared by reduction with sodium metal are significantly sensitive to heat and have high activities. For this reason, the particles partially undergo granulation at a considerably low temperature for the heat-treatment on the order of 1200.degree. to 1300.degree. C. This granulation leads to a decrease in the specific surface area of the particles and makes the production of a capacitor having a high capacity difficult. PA1 2 The decrease in the specific surface area may be inhibited by reducing the temperature of the heat-treatment, but the degree of oxygen-pickup is increased when removing the product from a heat-treating furnace after finishing the heat-treatment in proportion to the reduction in the temperature of the heat-treatment and the risk of ignition correspondingly increases. PA1 3 Moreover, a scattering (.+-.15.degree. C.) in the temperature is observed within the furnace for the heat-treatment and accordingly, there are likewise observed scatterings in physical properties (such as specific surface area, bulk, particle size distribution) and electrical properties (such as CV). PA1 4 The ingredient must be heat-treated little by little for preventing any ignition (for reducing the quantity of accumulated heat). In addition, it is necessary to remove the product from the furnace after completion of the heat-treatment while stepwise bringing it into contact with oxygen (several steps) and, for this reason, the productivity rate thereof is substantially impaired.